1. Field of the Invention
The present invention relates to an optical communication system and, more particularly, to a wavelength control method in an optical communication system which performs wavelength division multiplex communications.
2. Related Background Art
A wavelength division multiplex communication system can assign a large number of independent channels in a single transmission path. This communication system is suitable for a multimedia communication that requires flexibility of the network since it does not require any multiplex on the time axis such as frame synchronization or the like, and the respective channels need not have identical transmission rates.
As an example of the wavelength division multiplex communication system, a system in which each terminal station has a pair of wavelength variable optical transmitter and receiver is known. A transmitting terminal station adjusts the wavelength of its wavelength variable light source to a wavelength which is not used in communications (i.e., a "channel" in the wavelength division multiplex communication). On the other hand, a receiving terminal station receives a signal by matching the central wavelength of the passband spectrum of an optical band-pass filter (to be referred to as an optical filter hereinafter; the central wavelength will be referred to as the wavelength of the optical filter hereinafter) of its optical receiver to the wavelength to be received. The wavelength range that can be utilized by the system is determined by the wavelength variable range of the optical transmitter and receiver. The wavelength interval between adjacent channels (to be referred to as a channel interval hereinafter) is determined by the width (bandwidth or half-width) of the passband spectrum of the optical filter of the optical receiver.
As the wavelength variable light source, a wavelength variable semiconductor laser (the semiconductor laser will be referred to as an LD hereinafter) can be used. In order to broaden the wavelength variable width, various studies have been made. An LD which has a practical level at present is of multi-electrode DBR (distributed Bragg reflector) type, and has a wavelength variable width of several nm. For example, an LD described in OQE89-116 a "Three-electrode length resonator .lambda./4 shift MQW-DFB laser", The Institute of Electronics, Information and Communication Engineers, is known. On the other hand, as a wavelength variable filter, a Fabry-Perot resonator type filter can be used. At the current practical level, the wavelength variable range is several tens of nm, and the spectral width is about 0.1 nm. For example, a filter described in ECOC'90-605 "A field-worthy, high-performance, tunable fiber Fabry-Perot filter" is known.
In such system, by narrowing the channel interval, a larger number of channels can be assured while the wavelength variable remains the same.
In order to efficiently use a limited wavelength range, the respective channels are preferably assigned at a high density. As a technique for attaining such channel assignment, a technique described in U.S. Pat. No. 5,301,052 is known. In this technique, one reference light source is arranged in a network and outputs reference light, and the wavelengths used by transmitting stations are assigned at predetermined intervals in turn from that of the reference light. Each wavelength is controlled to have a predetermined wavelength interval from the neighboring wavelength. After the neighboring wavelength of a given wavelength becomes nondetectable, the wavelengths are controlled to shift so as to maintain the predetermined intervals. However, this technique does not take into consideration a case wherein the respective transmitting stations in the network have different outputtable wavelength ranges. In practice, however, it is not easy to manufacture light sources having the same wavelength variable range.